It is well known that, for isotactic polypropylene, the higher its isotacticity, the higher the crystallinity of the corresponding products. As a result, the mechanical properties such as hardness, rigidity, modulus, break strength and yield strength or the like are relatively good, and accordingly, the melting point, the thermal stability, aging resistance of the polymer are also relatively outstanding, however, the properties such as toughness, impact resistance, extension at break or the like will somewhat drop. The higher the isotacticity of the polypropylene, the narrower the molecular weight distribution and less benefitting the processing application and more difficult for injection molding and orientative film forming.
The methods for improving the processing properties of polypropylene include adjusting the isotacticity of polypropylene (e.g. Chinese Patent CN85101997) and widening the molecular weight distribution of polypropylene. As to product with wide molecular weight distribution, its fractions with high molecular weight provide good impact strength, modulus, melt strength and thermal property; and its fractions with low molecular weight provide processing flowability. Therefore, for polypropylene resins with the same grade number, polypropylene with wide molecular weight distribution has better rigidity, toughness and processing behavior, and has outstanding advantage in producing extrusion or injection molding article. Chinese Patent Application CN1156999A and Chinese Patent CN1137156C employed the method of mixing electron donor to adjust the molecular weight distribution of the polymers, while widened the molecular weight distribution, it increased the processing complexity, and accordingly, also increased production costs.